Neil Castle, Ph.D.
Douglas Krafte, Ph.D.
Aaron Gerlach, Ph.D.
Potassium channels are highly attractive as targets for the development of novel therapeutics. Their diversity and ubiquity, however, combined with a lack of detailed structural and functional insight, pose challenges for the development of selective drug candidates. Combining a multi-platform approach with advanced cell technology is helping to overcome some of these challenges. Advances in relevant high-throughput electrophysiology technologies are opening up opportunities for greater success.
Ubiquitous Physiological Involvement
Potassium channels are membrane proteins that form pores in cell membranes through which potassium ions (K+) flow. They are present in nearly all types of cells and involved in most physiological functions. There are in excess of 90 different types of potassium channels, which open and close in response to a range of signals (change in voltage, pH, ATP supply, intracellular calcium levels, etc.). Potassium channels activated by changes in cell membrane voltage (voltage-dependent potassium (Kv) channels) comprise the largest group. In humans, 40 genes have been identified that encode Kv channel subunits that can form homo- and hetero-multimeric channels, which are divided into 12 subfamilies.
The opening of potassium channels leads to the exit of K+ from cells and a drop in the resting membrane potential. As a result, K+ channels modulate nerve and muscle excitability, neurotransmitter and hormone release, water and electrolyte transport, cell proliferation and apoptosis, etc. Improperly functioning K+ channels have been associated with a number of diseases, including neurological and cardiovascular disorders, cancer, immune and metabolic diseases. Specific examples include epilepsy, diabetes, rheumatoid arthritis and multiple sclerosis (MS).
Several currently marketed drugs target potassium channels. For example, sulfonylurea drugs like Gliburide inhibit the Kir6 (KATP) class of K+ channels and have proven to be effective treatments for type 2 diabetes. The Kv inhibitor Dalfampridine (4-aminopyridine) has been clinically approved for the treatment of MS and the Kv7 activator Ezogabine (Retigabine) has been approved for treatment of epilepsy. Other K+ channel modulators are in late-stage preclinical development and are undergoing clinical trials for the treatment of diseases such as hypertension and psoriasis.
Selectivity is Key
Despite their importance, ion channels, and potassium channels in particular, have proved to be challenging drug discovery targets. The ubiquity of K+ channels makes it important to develop highly selective agents. For example, potassium channels belonging to the Kv7 family can be found in the heart and brain, where they play different roles in nerve excitability and cardiac muscle contractility. Targeting specific Kv7 channels in the brain to treat epilepsy, while avoiding modulation of Kv7 channels expressed in the heart, is critical to avoid unwanted cardiac toxicity. Even within the brain there are subtypes of Kv7 channels (i.e., Kv7.2/7.3 vs Kv7.3/7.5, Kv7.4), which potentially play different roles in disease and physiology, thus making subtype selective modulators of neuronal Kv7 channels desirable as drug development candidates.
Recognizing that specificity is important for K+ channel modulating drug candidates to be safe and efficacious, Icagen focuses on achieving selectivity early in the process. Our drug-discovery strategies are specifically designed to increase the likelihood of finding selective modulators that can be developed into successful drugs.
Our approach has involved cloning much of the ion channel genome in order to be able to generate a wide range of cell reagents that express many different channel classes, both human and species orthologues. In addition, we utilize continually evolving state-of-the-art electrophysiology and fluorescence assay platforms for the screening and characterization of agents, not only against channel members in the same family, but also other ion channels, enabling both target and off-target activity evaluation. We also regularly employ molecular biology to construct channel chimeras and mutants, which has enabled the identification of previously unknown drug binding sites on ion channels. Such knowledge of the correlation between binding site locations and enhanced selectivity can be applied during the development of other candidates for ion channel targets, and expands our ability to exploit potential interactions.
Applying Advanced Cellular Technologies
In combination with the platform approach described above, Icagen has also employed human induced Pluripotent Stem Cells (iPSC) as part of its integrated drug candidate development progression. The use of human tissues in drug development is important because it has been shown that the results obtained using animal tissues are not always a good indication of the drug’s performance in patients. Human iPS cells can be converted into a wide variety of cell types, including neurons and cardiac muscle, which allows for evaluation of drug candidates on actual human tissue. Furthermore, iPS cells can be obtained from human subjects carrying disease-associated genetic variants, which has opened up opportunities to assess not only the impact of the mutation on cell function but also drug candidate effects.
Thus drug candidate characterization is not limited to healthy human cells, but also to those carrying rare ion channel mutations observed in <1% of the population, as well as those present in a much larger percentage of the population. For example, we are able to determine if there are differences in the susceptibility for epilepsy or sensitivity to pain related to the presence of variants. This approach falls in line with the growing interest in precision/personalized medicine.
A Look at Kv7 (KCNQ) Modulators
A good example of Icagen’s utilization of integrated platform of technologies, including human iPS cell-derived neurons, can be found in our work developing activators of the Kv7 family of voltage-gated potassium channels. Genetic variants of these voltage-dependent ion channels, which are involved in membrane potential stabilization, action potential repolarization and modulation of neuronal bursting patterns, are linked to various forms of early onset epilepsies such as benign familial neonatal convulsions (BFNC).
The Kv7 family consists of five members that generally are closed in the resting state and open in response to depolarization of the cell membrane, due to excitatory synaptic inputs or by action potentials. The subunits Kv7.2 through Kv7.5 are most highly expressed in the nervous system, with mutation of Kv7.2 and Kv7.3 being genetically linked most frequently to epilepsy. When activated, Kv7 channels quiet neurons, making it more difficult to achieve electrical excitability in the brain. The key to controlling seizures is to tune back neuronal excitability to the appropriate level.
Most current drugs for the treatment of epilepsy lack selectivity and thus have a narrow therapeutic index. Unlike Kv7.2-7.5, the Kv7.1 channel is found in the heart and other tissues, but not in the nervous system; as such, it is the most structurally related liability target. There are multiple rare disease versions of genetically acquired epilepsy that are related to the loss of Kv7 channel function.
Developing Subtype Selective Channel Openers
Retigabine was the first Kv7.x activator to be developed for treatment of epilepsy. It functions as a pan activator of all Kv7 channel variants (Kv7.2/7.3, Kv7.3/7.5, Kv7.4, etc.) in the CNS. Current genetic information indicates that Kv7.2/7.3 channels are most commonly associated with hereditary epilepsy, and thus selective activation of this member of the Kv7 family of potassium channels may provide advantages over pan activators. For example, Kv7.4 plays an important role in auditory function and an activator may lead to unwanted side effects. Thus, selective Kv7.2/7.3 activators may have the advantage of a potentially lower side-effect profile.
Icagen was the first company to identify and develop subtype selective Kv7.2/7.3 activators. This was achieved by identifying drug candidates that interact with a previously unknown binding site on the voltage sensor of Kv7.x channels. This class of agents, exemplified by ICA-27243 and ICA-69673, were able to distinguish between Kv7.2/7.3 and Kv7.3/Kv7.5 channel subtypes while also being selective over Kv7.4 and the cardiac Kv7.1 family members. ICA-69673 advanced to human clinical trials; however, a non-target-related toxicological profile prevented further development. Nonetheless, the rationale for developing selective Kv7.x activators for treatment of neuroexcitatory disorders like epilepsy, amyotrophic lateral sclerosis (ALS) and pain remains.
While Retigabine is currently marketed to treat epilepsy, it is not widely used, possibly due to its side-effect profile. This inadequacy highlights the continuing need for more selective and effective Kv7 modulators. With access to more structural information on ion channels, effective high-throughput physiology testing techniques, advanced in silico predictive tools and improved models and assays, it is now possible to screen much larger libraries of compounds in order to identify more selective agents with better drug-like properties.
Leveraging Icagen’s Expertise
With over 20 years of experience in the development of drug candidates targeting ion channels, Icagen has the tools, expertise and experience needed to help partnering pharmaceutical and biotech companies achieve their drug development objectives. In addition to having the technology platforms to support current drug discovery progression, Icagen scientists have experience taking ion channel drug candidates into the clinic, including two activators of Kv7 potassium channels. Icagen also developed Senicapoc, a selective inhibitor of the KCa3.1 calcium-activated K+ channel that was assessed in phase III clinical trials for the treatment of sickle cell anemia and phase II clinical trials for asthma, and is currently being assessed for future clinical trial(s) for Alzheimer’s disease. Working with large pharma partners, Icagen scientists have also advanced several selective sodium channel inhibitors into clinical trials for treatment of pain and have worked with other companies to develop a cardiac Kv1.5 inhibitor for treatment of atrial arrhythmias, a calcium-activated potassium channel modulator for the treatment of memory and learning disorders, and Kir6 (KATP) channel openers for the treatment of urinary incontinence. We are eager to apply our experience in the identification and development of ion channel modulators to aid current and future internal and client-sponsored drug development programs.